You will have to call, you can ring 0800 800 150 and speak with the Value or Loyalty team.
Chances are you might have to wait until the 26th, if they were only just ordered the order could still be open, so it can't be cancelled until closed, so long as you do so within 30 days of ordering you won't be charged and if you are you would be automatically refunded as bills are generated in advance.